Variable stiffness heating catheter

ABSTRACT

The variable stiffness heating catheter includes a heating catheter shaft including at least one electrically conductive member, a reinforcing tube with apertures formed around the surface of the reinforcing tube, and at least one coaxial outer layer of a polymer, metal, or both for providing desired variations in stiffness along at least a portion of the length of the shaft. The apertures can be formed as axial or helical slits in the surface of the reinforcing tube, and the reinforcing tube can also be formed to be tapered at the point where the apertures are formed in the reinforcing tube to provide a heating catheter that is torqueable and pushable at the proximal end, yet soft and flexible at the distal end.

RELATED APPLICATIONS

This is a continuation-in-part of Ser. No. 09/275,485 filed Mar. 24,1999, now U.S. Pat. No. 6,352,531.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to interventional medical devices, andmore particularly concerns a heating catheter having variable stiffnessfor enhanced performance of the catheter shaft when used with or withouta guide catheter, as a part of a therapeutic system or for delivery ofmedical devices.

2. Description of Related Art

Conventional minimally invasive catheter based therapies typicallyrequire guide wires that are one to two meters long extending through alongitudinal lumen in the catheter, and that are torqueable and pushableat the proximal end, yet soft and flexible at the distal end. Many suchguidewires are made of stainless steel or the like, and are ground totapers which provide the desired bending properties along the guidewire.Recently, numerous minimally invasive sensing and actuation procedureshave been developed which benefit from the qualities of fiber optics todeliver optical light or power to the distal tip of the fiber optic.However, conventional fiber optic technology has not been easilyadaptable to such applications, particularly when the fiber optic mustalso act as a guidewire, either within a catheter or as a stand-alonedevice, since fiber optics, when used alone, are not very torqueable,pushable or resilient when compared to guide wires made from a varietyof other, more rigid, materials. Also, small diameter fiber optics arequite “floppy”, while larger diameter fibers can be too stiff tomaneuver through sharp bends, and the use of fiber optics as guidewiresor pushers within catheters can thus be difficult and quite techniquesensitive.

An abdominoscope is known that includes a tubular sheath having a seriesof strips separated by longitudinal slots, and an elongate, steerable,flexible medical implement is also known that has a tubular body with acontrollable steering region formed from flexible steering ribbons madeof flexible materials, such as Nitinol, spring steel, nylon, or otherplastic material. In addition, a steerable medical probe is also knownthat has a torque tube with spaced apart slots to impart additionalflexibility to the torque tube, with a thin-walled connecting portionserving as a rib or backbone. However, there remains a need for a way ofcreating variable stiffness along a heating catheter, other than a fiberoptic for example, without a decrease in the torquability andpushability of the heating catheter shaft.

It would also be desirable to provide a heating catheter shaft withvariable stiffness to allow such heating catheters to be more pushableat the proximal end and more trackable at the distal end, and to makethe use of heating catheters in catheter-based therapies morestraightforward and less technique sensitive. The present inventionaddresses these and numerous other needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for avariable stiffness heating catheter shaft formed from a heating catheterand a reinforcing tube over at least a portion of the heating catheter,with apertures being formed around the surface of the reinforcing tubeand extending in a direction between the proximal and distal ends of theheating catheter, to provide variable stiffness to the heating cathetershaft. Typically, such a heating catheter shaft can be formed from oneor more electrically conductive members or the like which alone havephysical characteristics that are undesirable for guidewires or pusherdevices. By use of the invention, a variable stiffness heating cathetershaft can be made which is more pushable at the proximal end and moretrackable at the distal end, with the capability to provide a wide rangeof predictable variations in stiffness and other structural parametersover the length of the shaft. A variable stiffness heating catheterconstructed according to the invention can be used in conjunction with aguide catheter or as a flow directed, stand alone catheter.

By using the construction according to the invention, coating or heatshrinking a heat shrinkable material on the outside diameter of theheating catheter shaft will improve tracking of the device, and a tapercan also be ground onto the heating catheter shaft to yield a shaft witha stiffer, more manageable, proximal end and a softer, moremaneuverable, distal tip. The variable stiffness heating catheteradvantageously can also thus be constructed from a minimum number ofcomponents, with the apertures in the reinforcing tube eliminating theneed for a braid or transitional sections from the stiffer proximal zoneto the softer distal zone.

The invention accordingly provides in a presently preferred embodimentfor a variable stiffness heating catheter for use in vascularinterventional therapy, such as for use within a tortuous, smalldiameter vessel such as those found in the vasculature of the brain. Thevariable stiffness heating catheter comprises at least one electricallyconductive member having a proximal end and a distal end, a reinforcingtube attached to the at least one electrically conductive member, withthe at least one electrically conductive member extending through thereinforcing tube, and the reinforcing tube having a surface defining aplurality of apertures extending in a direction between said proximaland distal ends of said heating catheter, and at least one coaxial outerlayer of a polymer, metal, or both provided over at least a portion ofthe reinforcing tube and the at least one electrically conductivemember, for providing desired variations in stiffness along at least aportion of the length of the shaft. In a presently preferred embodiment,the one or more electrically conductive members can be a pair of suchelectrically conductive wires. The reinforcing tube is preferably ametal tube, such as a hypo tube, and can be formed of stainless steel oran alloy of nickel and titanium, for example.

In one presently preferred embodiment, the apertures can be formed aslongitudinal, axial slits, slots, channels, or grooves in the surface ofthe reinforcing tube, and in an alternate preferred embodiment, theapertures can be formed as helical or radial slits, slots, channels, orgrooves in the surface of the reinforcing tube, providing variablestiffness to the heating catheter. The outer surface of the reinforcingtube can also be formed to be tapered at the point where the aperturesare formed in the reinforcing tube, particularly at a distal portion ofthe heating catheter, to provide a heating catheter that is torqueableand pushable at the proximal end, yet soft and flexible at the distalend. Alternatively, the apertures can be formed transversely in thesurface of the reinforcing tube in an area where such a configurationwill produce desired results.

The one or more coaxial layers can be formed of heat shrink polymericmaterial, such as polyethylene, polytetrafluoroethylene (PTFE)polyethylene terephthalate (PET), polyetherethylketone (PEEK),polyphenylenesulfide (PPS), or any of a variety of other polymers whichcan be fabricated into a structure and necked or shrunk over a shaft, orcan be formed of metal. While the invention can effectively use tubeswhich are placed over the exterior of the heating catheter shaft andthen heat shrunk or bonded by adhesive to the heating catheter shaft, itis also contemplated that the heating catheter shaft can be reinforcedby other longitudinally extending additional structures with varyingcross sections for certain specific applications.

The heat shrink tubing is placed on the heating catheter shaft, and thenheat can be applied to the heat shrink tubing, resulting in shrinkage ofthe heat shrink tubing to encapsulate the heating catheter shaft. Thestructure formed by the apertures in the surface of the reinforcingtube, in combination with the distal taper of the reinforcing tube andouter coaxial sheath, allows the proximal part of the composite shaft tobe relatively stiff, and the distal tip to be flexible and soft. Avariety of other techniques can be used within the scope of theinvention to accomplish the variable stiffness of the heating catheter.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings, which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a first preferred embodiment of avariable stiffness heating catheter according to the invention.

FIG. 1B is a schematic view of a second preferred embodiment of avariable stiffness heating catheter according to the invention.

FIG. 1C is a sectional view of a variant of the first preferredembodiment of the catheter shaft of FIG. 1A illustrating a pair ofelectrically conductive members.

FIG. 1D is a sectional view of a variant of the second preferredembodiment of catheter shaft of FIG. 1B illustrating a pair ofelectrically conductive members.

FIG. 2A is a schematic view of a third preferred embodiment of avariable stiffness heating catheter according to the invention.

FIG. 2B is a schematic view of fourth preferred embodiment of a variablestiffness heating catheter according to the invention.

FIG. 2C is a sectional view of a variant of the third preferredembodiment of the catheter shaft of FIG. 2A illustrating a pair ofelectrically conductive members.

FIG. 2D is a sectional view of a variant of the fourth preferredembodiment of catheter shaft of FIG. 2B illustrating a pair ofelectrically conductive members.

FIG. 3A is a schematic view of a fifth preferred embodiment of avariable stiffness heating catheter according to the invention.

FIG. 3B is a schematic view of a sixth preferred embodiment of avariable stiffness heating catheter according to the invention.

FIG. 3C is a schematic view of a seventh preferred embodiment of avariable stiffness heating catheter according to the invention.

FIG. 3D is a sectional view of a variant of the fifth preferredembodiment of the catheter shaft of FIG. 3A illustrating a pair ofelectrically conductive members.

FIG. 3E is a sectional view of a variant of the sixth preferredembodiment of catheter shaft of FIG. 3B illustrating a pair ofelectrically conductive members.

FIG. 3F is a sectional view of a variant of the seventh preferredembodiment of catheter shaft of FIG. 3C illustrating a pair ofelectrically conductive members.

FIG. 4A is a schematic view of an eighth preferred embodiment of avariable stiffness optical shaft according to the invention.

FIG. 4B is a schematic view of a ninth preferred embodiment of avariable stiffness optical shaft according to the invention.

FIG. 4C is a sectional view of a variant of the eighth preferredembodiment of catheter shaft of FIG. 4A illustrating a pair ofelectrically conductive members.

FIG. 4D is a sectional view of a variant of the ninth preferredembodiment of catheter shaft of FIG. 4B illustrating a pair ofelectrically conductive members.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Modern interventional medical procedures have relied on ever smaller andmore flexible devices to reach areas requiring treatment which werepreviously inaccessible to conventional devices, such as by theplacement of vasoocclusive devices in tiny areas of damaged vasculaturesuch as aneurysms or ruptures in arteries in the brain. Some devices totreat such areas use optical fibers to carry light energy to remotelocations at the distal end of the heating catheter, but certainlimitations have been found in currently available optical fibers forthose purposes.

For example, conventional heating catheter technology has not beeneasily adaptable to catheter based imaging, treatments such as“thrombolyzing” blood or cutting tissue, or to the delivery oftherapeutic agents, such as timed release agents, or embolics, sinceoptical fibers, when used as a stand alone structural device, are notvery torqueable, pushable or resilient. Small diameter optical fibers ofthe type most useful for such therapies frequently can become toofloppy, while larger diameter fibers can be too stiff to maneuverthrough sharp bends, and for these reasons, the use of optical fibers asstand alone guidewires or catheters can be difficult and techniquesensitive. Also, since there are practical limits to the diameter of thefiber for specific applications, the use of reinforced guide catheterswith longitudinal lumens through which the heating catheter passes canplace important restrictions on how small such an assembly can be.Further, if the heating catheter is to be used with both a guidewire anda guiding catheter, there are limits imposed on the techniques that canbe employed because of the necessarily larger diameter of such anassembly to accommodate the requirements of the two different shaftswithin the catheter.

As is illustrated in the drawings, which are provided for the purposesof illustration and not by way of limitation, one preferred embodimentof the invention illustrated in FIGS. 1A to 2D provides for a variablestiffness heating catheter 10 that comprises at least one electricallyconductive member 12 having a proximal end 14 and a resistive heatingelement (not shown) connected to the one or more electrically conductivemembers at a distal end 16. The at least one electrically conductivemember 12 is surrounded by a reinforcing tube 18, such as a metal tube,which can for example be a stainless steel hypo tube, although thereinforcing tube may also be formed of a nickel titanium alloy, such asNITINOL. The reinforcing tube can be cylindrical as shown in FIG. 1, ormay be tapered along its length, either in steps or continuously inorder to provide a desired stiffness and pushability. In one presentlypreferred embodiment, the reinforcing tube is advantageously providedwith a plurality of longitudinal, axially oriented apertures, such asslits, slots, channels, or grooves 20 formed around the surface of aportion of the reinforcing tube to provide a heating catheter that istorqueable and pushable at the proximal end, yet soft and flexible atthe distal end. As is illustrated in FIG. 1A, the apertures can beformed in the surface of the reinforcing tube, such as by laser cutting,for example, and allow the reinforcing tube to have the same diameter,or a tapering diameter, with variable stiffness. In a presentlypreferred alternate embodiment illustrated in FIG. 1B, in which likeelements are designated with like reference numerals, the apertures canbe formed as groups 20′ of round holes 21, arranged so as to belongitudinal, and axially oriented around the surface of a portion ofthe reinforcing tube, so as to provide a heating catheter that istorqueable and pushable at the proximal end, yet soft and flexible atthe distal end. As is illustrated in FIGS. 1C and 1D, the one or moreelectrically conductive members can be a pair of such electricallyconductive wires 12′, for example.

Alternatively, as is illustrated in FIG. 2A, in which like elements aredesignated with like reference numerals, the apertures can be formed inthe surface of the reinforcing tube by grinding. In another presentlypreferred aspect illustrated in FIG. 2A, the outer surface of thereinforcing tube can also be formed to have a taper 22 at the pointwhere the apertures are formed in the reinforcing tube, such as bygrinding or laser cutting, particularly at a distal portion of theheating catheter, to provide a heating catheter that is torqueable andpushable at the proximal end, yet soft and flexible at the distal end.In a presently preferred alternate embodiment illustrated in FIG. 2B, inwhich like elements are designated with like reference numerals, theapertures can be formed as groups 20′ of round holes 21, such as bylaser cutting, at a distal portion of the heating catheter, to provide aheating catheter that is torqueable and pushable at the proximal end,yet soft and flexible at the distal end. As is illustrated in FIGS. 2Cand 2D, the one or more electrically conductive members can be a pair ofsuch electrically conductive wires 12′, for example.

Referring to FIGS. 1A, 1B, 1C, 1D, 2A, 2B, 2C and 2D, in a currentlypreferred embodiment, at least one coaxial outer covering or sheath 24is also provided over at least a portion of the heating catheter andreinforcing tube, for providing desired variations in stiffness along atleast a portion of the length of the shaft. Such an outer sheath iscurrently preferably a heat shrink polymer, such as polyethylene, PTFE,PEEK, PET or PPS, for example, although other similar heat shrinkpolymers may also be suitable. Alternatively, the outer sheath can be ametal tube, such as a metal tube formed from a nickel titanium alloy,for example, that can be bonded adhesively over the outer surface of theheating catheter, such as by cyanoacrylate adhesive. Alternatively,where the reinforcing tube is sufficient to provide the desired variablestiffness characteristics to the heating catheter, the outer sheath maybe omitted.

In another presently preferred embodiment, illustrated in FIG. 3A, avariable stiffness heating catheter 30 is provided that comprises atleast one electrically conductive member 32 having a proximal end 34 anda resistive heating element (not shown) connected to the one or moreelectrically conductive members at a distal end 36. The at least oneelectrically conductive member 32 is surrounded by a reinforcing tube38, such as a metal tube, which can for example be a stainless steelhypo tube, although the reinforcing tube may also be formed of a nickeltitanium alloy, such as NITINOL. The reinforcing tube can be cylindricalas shown in FIGS. 1A and 1B, or may be tapered along its length, eitherin steps or continuously in order to provide a desired stiffness andpushability. At least one coaxial outer covering of sheath 24 is alsoprovided over at least a portion of the at least one electricallyconductive member and reinforcing tube, as discussed above. In onepresently preferred embodiment, the reinforcing tube is advantageouslyprovided with a plurality of helically arranged apertures 40 formed asslits, channels or grooves around the surface of a portion of thereinforcing tube to provide a heating catheter that is torqueable andpushable at the proximal end, yet soft and flexible at the distal end.Alternatively, the apertures may be radially arranged apertures formedas slits, channels or grooves.

In another presently preferred embodiment, illustrated in FIG. 3B, theapertures 40′ can be axially oriented, helically arranged aperturesformed as slits, channels or grooves. The apertures can be formed in thesurface of the reinforcing tube by grinding or laser cutting, and allowthe heating catheter to have a constant diameter, or a taperingdiameter, from the proximal to the distal end of the shaft. In apresently preferred alternate embodiment illustrated in FIG. 3C, inwhich like elements are designated with like reference numerals, theapertures can be arranged in a pattern of a varying density, such as ina gradient, of round holes 41, which can be formed such as by lasercutting, helically arranged so as to provide a heating catheter that istorqueable and pushable at the proximal end, yet soft and flexible atthe distal end. In another presently preferred aspect, the outer surfaceof the reinforcing tube can also be formed to have a taper (not shown)at the point where the helical apertures are formed in the reinforcingtube, such as by grinding or laser cutting, particularly at a distalportion of the reinforcing tube, to provide a heating catheter that istorqueable and pushable at the proximal end, yet soft and flexible atthe distal end. As is illustrated in FIGS. 3D, 3E and 3F, the one ormore electrically conductive members can be a pair of such electricallyconductive wires 32′, for example.

In a further aspect of the invention illustrated in FIG. 4A, in whichlike elements are designated with like reference numerals, all or aportion of the shaft 42 can be formed with alternating laterallyarranged apertures or cuts, slits, channels or grooves 44, 46 in thereinforcement tube to produce a composite shaft that has desiredflexibility in a specific area of the shaft. Such a configuration caninclude variable width and depth of the apertures 44, 46 in order tovary the flexibility, torqueability and pushability of the shaft. In apresently preferred alternate embodiment illustrated in FIG. 4B, inwhich like elements are designated with like reference numerals, thelaterally arranged apertures can be formed as groups 44′, 46′ of roundapertures 47, formed such as by laser cutting, for example. As isillustrated in FIGS. 4C and 4D, the one or more electrically conductivemembers can be a pair of such electrically conductive wires 32′, forexample.

As described above, at least one coaxial outer covering or sheath 24 isalso provided over at least a portion of the heating catheter and thereinforcing tube, for providing desired variations in stiffness along atleast a portion of the length of the shaft. Such an outer sheath iscurrently preferably a heat shrink polymer, such as polyethylene, PTFE,PEEK, PET or PPS, for example, although other similar heat shrinkpolymers may also be suitable. Alternatively, the outer sheath can be ametal tube, such as a metal tube formed from a nickel titanium alloy,for example, that can be bonded adhesively over the outer surface of theheating catheter, such as by cyanoacrylate adhesive.

The one or more coaxial layers can be formed of heat shrink polymericmaterial, such as polyethylene, polytetrafluoroethylene (PTFE)polyethylene terephthalate (PET), polyetherethylketone (PEEK, also knownas polyaryletherketone), polyphenylenesulfide (PPS), or any of a varietyof other polymers which can be fabricated into a structure and necked orshrunk over a shaft, or can be formed of metal. While the invention caneffectively use tubes which are placed over the exterior of the heatingcatheter shaft and then heat shrunk or bonded by adhesive to the heatingcatheter shaft, it is also contemplated that the shaft can be reinforcedby other longitudinally extending additional structures with varyingcross sections for certain specific applications.

The heat shrink tubing is placed on the heating catheter shaft, and thenheat can be applied to the heat shrink tubing, resulting in shrinkage ofthe heat shrink tubing to encapsulate the heating catheter shaft. Thestructure formed by the apertures in the surface of the reinforcingtube, in combination with the distal taper of the reinforcing tube andouter coaxial sheath, allows the proximal part of the composite shaft tobe relatively stiff, and the distal tip to be flexible and soft. Avariety of other techniques can be used within the scope of theinvention to accomplish the variable stiffness of the heating cathetershaft.

For neurovascular use, the overall length of a heating catheter pushercan be, for example, from 100 to 300 cm, with the outer diameter beingless than about 1 French (0.0135 inch). For peripheral use, the overalllength of the catheter can be, for example, from 100 to 300 cm, with theouter diameter of the distal 25 to 45 cm being less than about 5 French(0.063 inch), and the outer diameter of the proximal 100 cm being lessthan about 6 French (0.075 inch). For cardiovascular use, the overalllength of the catheter can be, for example, from 150 to 175 cm, with theouter diameter of the distal 25 cm being less than about 3 French (0.038inch), and the outer diameter of the proximal 100 cm being less thanabout 4 French (0.050 inch). These dimensions are approximate, and inpractical terms, depend upon sizes of shrink tubing that arecommercially available.

In practice, heating catheter shafts used for micro-coil delivery andthe like are approximately 0.003 to approximately 0.014 inches indiameter, with the outer buffer comprising a layer of approximately0.0005 to 0.002 inches in thickness of a polymer over a thin layer ofcladding used to limit the dissipation of light out of the shaft. In onepresently preferred embodiment, the outer buffer can be centerlessground to provide a variable thickness characteristic and the heatingcatheter shaft can be manufactured with a thicker than normal buffer tofacilitate grinding of the buffer to provide a desired bending stiffnesseither with or without additional layers of stiffening polymers over theouter surface of the heating catheter shaft.

In one example of the method of manufacturing the variable stiffnessheating catheter of the invention, the shaft can be assembled by slidingand centering a polyethylene coaxial heat shrink tube, which can be, forexample, 200 cm in length, over a heating catheter, which can be, forexample, 205 cm long. The ends of the heating catheter are then clamped,and tension is applied to keep the heating catheter taut. The proximalend of the polyethylene heat shrink tube is placed into the working areaof a heat gun, although other means of controllably heating the heatshrink polymeric sheath may be used. The temperature of the polyethyleneheat shrink tube is heated to approximately 650 F., and the rest of theheat shrink tube is heated by sliding the heat gun along the axis of theheat shrink tube at about three inches per second, for example, untilthe heat gun has traveled the length of the polymeric material and thepolyethylene has encapsulated the heating catheter shaft. This method isrepeated for 150 cm and 100 cm lengths of polymeric tubing, and anyfurther heat shrink tubing to be used for varying the stiffness of theheating catheter, until the outside diameter of the shaft is built up tothe desired dimensions to yield the desired degrees of stiffness.

Those skilled in the art will recognize that a variety of polymers,including those filled with reinforcing fibers or other material may beused to reinforce a heating catheter so that it can be more effectivelyused as a pusher within a catheter lumen or as a free therapeuticmember. For example, the characteristics of the materials to be used maybe optimized by use of abutting adjacent covers of different materialsagainst one another longitudinally in end to end fashion to thus providea constant outer diameter. In such a construction, the outer sheath isjoined by heat and/or pressure or adhering bonded sections surroundingspecific portions of the heating catheter, to provide a smooth overallexterior to the finished composite shaft.

It will be apparent from the foregoing that while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. For example, some of the various techniques of the inventioncan be advantageously combined for certain applications, while othersare effectively met by only one aspect of the embodiments discussed.Accordingly, it is not intended that the invention be limited, except asby the appended claims.

1. A variable stiffness heating catheter for use in interventionalvascular therapy, comprising: a heating catheter shaft having a proximalend and a distal end, said heating catheter shaft including at least oneelectrically conductive member for providing resistive heating; areinforcing tube attached to the heating catheter shaft, the heatingcatheter shaft extending through said reinforcing tube, and saidreinforcing tube having a surface defining a plurality of apertures toprovide variations in stiffness along the length of the heating cathetershaft; and at least one outer coaxial sheath over at least a portion ofsaid heating catheter shaft and covering said plurality of apertures ofsaid reinforcing tube.
 2. The variable stiffness heating catheter ofclaim 1, wherein said plurality of apertures comprise axial slits formedin the surface of the reinforcing tube.
 3. The variable stiffnessheating catheter of claim 1, wherein the apertures can be formed ashelical slits in the surface of the reinforcing tube.
 4. The variablestiffness heating catheter of claim 1, wherein the outer surface of thereinforcing tube is tapered along its length.
 5. The variable stiffnessheating catheter of claim 1, wherein the apertures comprise a pluralityof lateral slits formed in the surface of the reinforcing tube.
 6. Thevariable stiffness heating catheter of claim 4, wherein the outersurface of the reinforcing tube is tapered at the point where theapertures are formed in the reinforcing tube.
 7. The variable stiffnessheating catheter of claim 1, wherein the outer surface of thereinforcing tube is tapered at a distal portion of the reinforcing tube,whereby said heating catheter is torqueable and pushable at the proximalend, yet soft and flexible at the distal end.
 8. The variable stiffnessheating catheter of claim 1, wherein said outer coaxial sheath is formedfrom a material selected from the group consisting of a polymer, metal,or a combination thereof.
 9. The variable stiffness heating catheter ofclaim 8, wherein said polymer comprises heat shrink polymeric material.10. The variable stiffness heating catheter of claim 9, wherein saidpolymer is selected from the group consisting of polyethylene,polytetrafluoroethylene, polyethylene terephthalate,polyetherethylketone, and polyphenylenesulfide.
 11. The variablestiffness heating catheter of claim 1, wherein said at least oneelectrically conductive member comprises a pair of electricallyconductive wires.
 12. A variable stiffness heating catheter for use ininterventional vascular therapy, comprising: a heating catheter shafthaving a proximal end and a distal end, said heating catheter shaftincluding at least one electrically conductive member for providingresistive heating; a reinforcing tube attached to the heating cathetershaft, the heating catheter shaft extending through said reinforcingtube, and said reinforcing tube having a surface defining a plurality ofapertures extending in a direction between said proximal and distal endsof said heating catheter shaft; and at least one outer coaxial sheathover at least a portion of said heating catheter shaft and covering saidplurality of apertures of said reinforcing tube, to provide variationsin stiffness along the length of the heating catheter shaft.
 13. Thevariable stiffness heating catheter of claim 12, wherein said pluralityof apertures comprise axial slits formed in the surface of thereinforcing tube.
 14. The variable stiffness heating catheter of claim12, wherein the apertures can be formed as helical slits in the surfaceof the reinforcing tube.
 15. The variable stiffness heating catheter ofclaim 12, wherein the outer surface of the reinforcing tube is taperedalong its length.
 16. The variable stiffness heating catheter of claim12, wherein the apertures comprise a plurality of lateral slits formedin the surface of the reinforcing tube.
 17. The variable stiffnessheating catheter of claim 15, wherein the outer surface of thereinforcing tube is tapered at the point where the apertures are formedin the reinforcing tube.
 18. The variable stiffness heating catheter ofclaim 17, wherein the outer surface of the reinforcing tube is taperedat a distal portion of the reinforcing tube, whereby said heatingcatheter is torqueable and pushable at the proximal end, yet soft andflexible at the distal end.
 19. The variable stiffness heating catheterof claim 12, wherein said outer coaxial sheath is formed from a materialselected from the group consisting of a polymer, metal, or a combinationthereof.
 20. The variable stiffness heating catheter of claim 19,wherein said polymer comprises heat shrinkable polymeric material. 21.The variable stiffness heating catheter of claim 20, wherein saidpolymer is selected from the group consisting of polyethylene,polytetrafluoroethylene, polyethylene terephthalate,polyetherethylketone, and polyphenylenesulfide.
 22. The variablestiffness heating catheter of claim 12, wherein said at least oneelectrically conductive member comprises a pair of electricallyconductive wires.